Modern aircraft generally have a synthetic vision system, referred to by the acronym “SVS”. This system makes it possible to present to the crew a synthetic image of the outside landscape generally including piloting or navigation information.
An SVS system comprises a cartographic database representative of the terrain being flow over, a geolocation system, electronic computation means and several display devices located in the cockpit of the aircraft. The geolocation system is of the “GPS” type, GPS being the acronym for “Global Positioning System”. It can be coupled with the inertial system of the aircraft. The geolocation system as a whole supplies at least the following parameters: position of the aircraft in attitude, longitude and altitude and orientation of the aircraft in pitch, roll and heading.
Generally, the image is displayed on the display screens which are located on the front panel of the aircraft instrument panel. The image is a three-dimensional view of the outside represented as realistically as possible. The point of view displayed is in the axis of the aircraft. The synthetic image is computed to a certain distance from the aircraft so as to limit the computations necessary to the display. This distance is called “SVS range”. In effect, beyond a certain distance, the image of the landscape is of small dimensions. Moreover, it is of little interest for the piloting of the aircraft.
This synthetic image generally comprises a piloting and navigation aid symbology. It conventionally comprises an artificial horizon giving the attitude of the aircraft and indicators giving the altitude and the speed of the aircraft. This symbology also displays a line representing the zero longitudinal pitch indicator, also called “ZPRL”, the acronym for “Zero Pitch Reference Line”. In an abuse of language, the “ZPRL” is often called “horizon line”.
As can be seen in FIG. 1 which represents a view in vertical cross section of an aircraft A flying over a terrain T, the ZPRL forms, with the true horizon line LH, a first angle α. This line forms, with the limit of the cartographic representation RC, a second angle β that is necessarily greater than the first angle. These angles are generally several degrees at high altitude.
FIG. 2 represents the display, on an aircraft display device, of a cartographic representation of the terrain being flown over comprising piloting symbology. This symbology comprises a ZPRL. In this figure, the difference between this ZPRL and the end of the cartographic representation is notable. It has been demonstrated that a significant angular deviation between the SVS range and the ZPRL is very disturbing for the pilots because it does not correspond to the usual visual orders of magnitude. That occurs above all at high altitude, the deviation increasing with the altitude of the aircraft.
Moreover, some aeronautical standards like the standard AC 20-167 entitled “Airworthiness Approval of Enhanced Vision System, Synthetic Vision System, Combined Vision System, and Enhanced Flight Vision System Equipment”, require the information provided by the SVS to be correlated with the ZPRL. For example, the terrain which is located above the altitude of the carrier at a given instant must always appear above the ZPRL, if it is close enough to be dangerous.
A first means for resolving these various problems is to display a cartographic representation over longer distances. The defect with this solution is a significant additional requirement in terms of performance of the electronic platform both at the central processor level and at the graphical computation means level to allow the display of the image at the expected frequency and image quality.
A second means consists in simplifying the representation of the terrain as a function of the distance from the terrain to the aircraft. The U.S. Pat. No. 6,862,501 entitled “Method for producing 3D perspective view avionics terrain displays” and the U.S. Pat. No. 7,262,713 entitled “System and method for a safe depiction of terrain, airport and other dimensional data on a perspective flight display with limited bandwidth of data presentation” propose solutions of this type.